An electrolyte is used in (aqueous) solutions in many cases. It is, however, a recent tendency to replace a solution electrolyte with a solid electrolyte. A first reason for this tendency is ease of processing when an electrolyte is used for electric and electronic materials, for example, and a second reason is shifting to small and light materials and laborsaving. Proton conductive materials include inorganic materials and organic materials. As an example of inorganic electrolytes, a hydrate compound such as uranyl phosphate can be given. An inorganic compound, however, does not provide a sufficient interface and has many problems when a conduction layer is formed on a substrate or an electrode.
On the other hand, as examples of organic compounds, polymers belonging to a cation exchange resin, such as a sulfonated product of a vinyl polymer such as polystyrene sulfonic acid, a perfluoroalkyl sulfonic acid polymer represented by Nafion™ (manufactured by E.I. du Pont de Nemours and Company), a perfluoroalkyl carboxylic acid polymer, and a polymer produced by introducing a sulfonic acid group and a phosphoric acid group into a heat resistant polymer such as polybenzimidazole and polyether ether ketone (see, for example, Polymer Preprints, Japan, 2490-2942, Vol. 42, No. 7 (1993), Polymer Preprints, Japan, 735-736, Vol. 43, No. 3 (1994), and Polymer Preprints, Japan, 730, Vol. 42, No. 3 (1993)) can be given.
These organic polymers are usually used in the form of a film. Due to their capability of being soluble in a solvent and their thermoplastic properties, organic polymers can be processed by joining with a conductive film on an electrode. However, due to insufficient proton conductivity and poor durability, many of these organic polymers have problems such as a decrease in the proton conductivity at a high temperature (100° C. or more), a significant decrease in the mechanical properties (particularly, modulus of elasticity), large dependability under humid conditions, not necessarily sufficient adhesion with an electrode, and a decrease in the strength and deformation due to excessive swelling during use which is caused by the hydrated polymer structure. Therefore, these organic polymers have various problems in applying to the electric and electronic materials and the like.
U.S. Pat. No. 5,403,675 proposes a solid polyelectrolyte made from sulfonated rigid polyphenylene. The solid polyelectrolyte comprises a polymer obtained by polymerizing an aromatic compound having a phenylene chain (the structure described in column 9 of U.S. Pat. No. 5,403,675) as a main component, into which a sulfonic acid group is introduced by reacting with a sulfonation agent. Although the proton conductivity is improved by increasing the amount of sulfonic acid groups introduced, the mechanical properties of the resulting sulfonated polymer, for example, breaking elongation, toughnesses such as bending resistance, and hot water resistance are remarkably impaired.